search for unknown operator 'BPY_OT_OPS_OT_mesh.dissolve_limited()
I tried your script and had a few other issues aside from the one you mentioned. I tweaked it to make it work and had no issues. I bet you're getting the search for unknown operator from something else. Try going to File &gt; Load Factory Settings to reset Blender.

import bpy
import os
import operator


class CustomPanel(bpy.types.Panel):
    bl_label = "OpenSource::Classicism XL"
    bl_space_type = "SEQUENCE_EDITOR"
    bl_region_type = "UI"
    bl_options = {'DEFAULT_CLOSED'}

    def draw(self, context):
        row = self.layout.row()
        row.label("Intro Directory")

        col = self.layout.split().column(align = True)
        col.operator("sequencer.add_intro" , text= "Import")


class Add_Intro(bpy.types.Operator):
    bl_idname = "sequencer.add_intro"
    bl_label = "Add Intro"

    def execute(self, context):

        def add_splash_screen():
            bpy.ops.sequencer.image_strip_add(directory="/home/doakey/Desktop/image.png", files=[{"name":"image.png"}], frame_start=1, frame_end=26, channel=1)

        add_splash_screen()

        return {'FINISHED'}


def register():
    bpy.utils.register_module(__name__)


def unregister():
    bpy.utils.unregister_module(__name__)


if __name__ == "__main__":
    register()